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Bacteria are one of the most important life-forms on Earth. Being one of the first organisms to have appeared on the planet, they have evolved to thrive in a variety of environments. Peritrichous (multi-flagellated) bacteria, such as T. majus or E. coli, self-propel in fluids by using specialized rotary motors to rotate multiple helical filaments, enabling swimming of the whole cell. In the present work, we focus on the surface interactions mechanisms of bacteria such as T. majus that have a large spherical cell body compared to their flagellar length, in contrast with E. coli that have a relatively smaller but elongated prolate-spheroidal cell body.

In recent experiments, the swimming behavior of T. majus cells was studied near surfaces. Quite surprisingly, it was found that instead of swimming along the surface in circles (as well established for many species), many freely swimming cells became dynamically surface-bound. In this bound state, cells remained free to move laterally along the surface and their bodies continuously rotated around their center in the direction perpendicular to the surface while their flagellar filaments pointed away from the surface, rotating in the opposite direction. The bound state is in stark contrast to the classical situation of bacteria swimming in circular paths near surfaces. The question therefore arises: what is the mechanism at the origin of this transition to a bound state? In the bound state, a small perturbation in the tilt angle of the flagellum is expected to destabilize the cell and cause it to swim parallel to the surface in circles. What exactly makes this state stable? Using a combination of theory and simulations, we show that the transition from swimming to a bound state can be rationalized as an instability due to fluid-structure interaction.

We then focus on the question: how does changing the shape of the cell body from a sphere to a prolate spheroid change these swimming dynamics near surfaces? Quite surprisingly, we find that on simply changing the shape of the cell body from a sphere (relevant for T. majus) to a prolate spheroid (relevant for E. coli), the cells swimming parallel to the surface in circular trajectories do so at a finite height above the surface of magnitude similar to the width of the body. This is a rather surprising result as bacteria are force-free organisms and, as force-dipoles swimming in a fluid medium, they should always be attracted towards no-slip surfaces. The development of a simple mathematical model to explain these curious findings is under progress.